Biodegradable microparticles are one of the most studied delivery devices in medicine. Microparticles were developed to circumvent problems concerning the fragility and short in vivo half-lives of protein-based drugs. These compounds differ from traditional small-molecule drugs since oral administration usually results in the destruction of proteins and peptides during digestion.
The medical applications of microencapsulation via liposomes, microspheres, and colloids are expanding rapidly. In general, colloidal formulations may provide alternative pharmacological properties to pharmacological agents. In many cases, a drug with low water solubility is formulated as a colloidal emulsion (with a high concentration of the active agent) to deliver the required dose more efficiently. Emulsions, liposomes, and solid aggregates are frequently formed using detergents, phospholipids, or polymeric materials. Liposomes, the most generally studied medical colloid, consist of phospholipid bilayers held together only by weak, non-covalent hydrophobic interactions. Liposomes, however, can encapsulate only aqueous solutions and only up to concentrations limited by osmolarity. Because such colloids are held together only be weak intermolecular interactions, they generally have limited shelf-lives, and more critically, are prone to changes in aggregation state in vivo.
The emerging practical potential of protein microparticles has been realized recently, however. Albunex® is an FDA-approved, air-filled albumin microparticle produced ultrasonically that is used intravenously as a contrast agent for ultrasound imaging and as an echo-contrast agent for echocardiography [11–13]. These microparticles may also be formed with encapsulated liquid, to form a unique colloidal delivery vehicle. By the choice of protein used for the microparticle shell and the material encapsulated within the microparticle, a multitude of biomedical applications have been developed [12, 14–18]. Some of the applications of microparticles include biocompatible blood substitutes, magnetic resonance imaging and echocardiographic contrast agents, and novel drug delivery systems. These are described in the following U.S. Pat. Nos. 5,362,478; 5,439,686; 5,498,421; 5,505,932; 5,508,021; 5,512,268; 5,560,933; 5,635,207; 5,639,473; 5,650,156; 5,665,382 and 5,665,383.
Ultrasonic irradiation of aqueous protein solutions results in the creation of microparticles having a protein shell. Studies have delineated that the mechanism responsible for microparticle formation is a combination of two acoustic phenomena: emulsification and cavitation. Ultrasonic emulsification creates the microscopic dispersion of the protein solution necessary to form the shape of the proteinaceous microparticle shell. Emulsification alone is insufficient to produce long-lived microparticles. This is attributed to the fact that the interactions between protein subunits that maintain the proteins within the microparticle shell's architecture are not strong enough to overcome entropy-driven dissociation. For example, while emulsions generated by vortex mixing produce microparticles, these particles will disassemble into individually solvated protein components over time.
Ultrasonic irradiation of liquids can also produce cavitation, which is the formation, growth, and implosive collapse of bubbles. The collapse of such bubbles creates transient hot-spots with enormous peak temperatures [9]. Sonolysis of water is known to produce a variety of reactive species, such as H+, OH−, H2, H2O2, and in the presence of oxygen, HO2 [10]. These species recombine to form water, escape from solution as gas, or undergo further reaction among themselves and with other solution components. Among the various reactions that occurs with sonolysis of protein solutions is the formation of inter-protein crosslinked products. Sonolysis-produced superoxide creates inter-protein disulfide bonds that cross-link the protein components of a microparticle, thereby imparting the requisite stability necessary for maintaining the microparticle shell's architecture over time. Thus, the dispersion of gas or nonaqueous liquid into the protein solution to create a microparticle shell, coupled with chemical cross-linking of the protein at the microparticle interface, results in the formation of long-lived microparticles filled with air or liquid.